Hydraulic architecture parameters with relative ramification rate and in different area stem segments under ambient and elevated CO₂ concentrations were measured by improved flushing method. The aims were to study the change of hydraulic architecture, hydraulic safety and efficiency under elevated CO₂ concentration. Saplings of four tree species (Pinus tabuliformis, Platycladus orientalis, Acer truncatum and Robinia pseudoacacia) had been exposed to ambient and elevated CO₂ concentration for 13 months. The results showed that xylem specific conductivity (K_s) and leaf specific conductivity (LSC) increased in all the ramifications under elevated CO₂ concentration. However, the percent loss of hydraulic conductivity (PLC) decreased in P. orientalis and R. pseudoacacia in all the ramifications, and PLC only decreased in branch 2 of P. tabuliformis and in branch 0 and 1 of A. truncatum. K_s increased and PLC decreased in branch 2 of P. tabuliformis and R. pseudoacacia. In P. orientalis, P. tabuliformis and R. pseudoacacia, K_s increased by 12.8%, 19.6% and 51.24% in branch 2 whereas PLC decreased by 11.80%, 9.6% and 51.01% under elevated CO₂ concentration, respectively. The hydraulic conductivity (K_h) of P. tabuliformis and P. orientalis in the non-restricted area were respectively larger than that in the restricted area, whereas K_h of A. truncatum and R. pseudoacacia were respectively smaller than that in the restricted area. However, the PLC was not significantly affected by elevated CO₂ concentration. K_s and LSC of P. orientalis and A. truncatum in the nonrestricted area were respectively larger than those in the restricted area, whereas K_s and LSC of P. tabuliformis and R. pseudoacacia were respectively smaller than those in the restricted area. Under elevated CO₂ concentration, four tree species with relative ramification showed different strategies to adapt to the changed environment. The strategies included: increasing hydraulic safety and hydraulic efficiency, and decreasing hydraulic security to tradeoff the hydraulic efficiency. However, four tree species in different area stem segments showed different strategies, keeping higher hydraulic efficiency to adapt to the elevated CO₂ concentration.